A control device described below is known as a control device of a secondary battery. The control device defines a predetermined battery model, estimates a terminal voltage of the secondary battery which is based on the battery model, as an estimated voltage value on the basis of measured values of a current and a terminal voltage of the secondary battery, sequentially identifies parameters of the battery model in such a way that the difference between the measured voltage value and the estimated voltage value converges to zero, and thereby estimates the state inside the battery (see, for example, Patent Literature 1).